Fix invalid use of StringRef::data in Socket::DecodeHostAndPort

the input StringRef is not guaranteed to be null-terminated, so using
data to get the c string is wrong. Luckily, in two of the usages the
target function already accepts a StringRef so we can just drop the
data() call, and the third one is easily replaced by a stringref-aware
function.

Issue found by msan.

llvm-svn: 355817